1. Field of the Invention
This invention relates to a fuel injection control system for an internal combustion engine of an in-cylinder fuel injection type which is operated while switching between a stratified combustion mode in which fuel injection into each cylinder is performed during a compression stroke and a homogenous combustion mode in which the same is performed during an intake stroke, the fuel injection control system controlling the timing of the fuel injection.
2. Description of the Prior Art
Conventionally, a fuel injection control system of the above-mentioned kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 8-42381. In this fuel injection control system, fuel injection timing in the stratified combustion mode is calculated as follows: First, based on fuel pressure and a required fuel amount, a valve opening time period (fuel injection time period) is determined over which a fuel injection valve is required to be opened for injecting the required amount of fuel. Then, termination timing of the fuel injection timing is determined such that the fuel injection is terminated during a compression stroke. Based on the termination timing and the fuel injection time period, start timing of the fuel injection timing is calculated. Further, a correction term is calculated based on an engine coolant temperature, an engine rotational speed, and an engine load, and the correction term is added to the fuel injection timing to thereby determine a final fuel injection timing.
In general, in the in-cylinder fuel injection combustion engine, fuel is directly injected into a combustion chamber, and therefore, to obtain a desired engine output, it is required to accurately control not only the fuel injection time period but also the fuel injection timing. To this end, the conventional fuel injection control system calculates the fuel injection time period based on the fuel pressure and the required fuel amount, and corrects the fuel injection timing determined from the fuel injection time period by using the correction term determined depending on the engine coolant temperature, the engine rotational speed, and the engine load. In the calculation of the fuel injection time period, however, in order to compensate for influence of a dynamic behavior of fuel, such as deposition of fuel, and influence of purge of evaporative fuel, it is desirable to correct the fuel injection time period by using parameters indicative of such behaviors of fuel. On the other hand, when such a correction of the fuel injection time period is carried out, the conventional fuel injection control system cannot properly calculate the fuel injection timing since the correction thereof dependent on the above-mentioned parameters is not carried out. This can degrade drivability due to lowered engine output, as well as fuel economy.
It is an object of the invention to provide a fuel injection control system for an internal combustion engine, which is capable of properly determining fuel injection timing such that the fuel injection timing reflects the behaviors of injected fuel, thereby enabling improvement of drivability and fuel economy.
To attain the above object, according to a first aspect of the invention, there is provided a fuel injection control system for an internal combustion engine of an in-cylinder fuel injection type which is operated while switching between a stratified combustion mode in which fuel injection into each cylinder is performed during a compression stroke and a homogenous combustion mode in which the fuel injection into the cylinder is performed during an intake stroke, the fuel injection control system controlling timing of the fuel injection.
The fuel injection control system according to the invention is characterized by comprising:
operating condition-detecting means for detecting operating conditions of the engine;
required fuel amount-determining means for determining a required fuel amount based on the detected operating conditions of the engine;
parameter-calculating means for calculating parameters indicative of behaviors of the fuel based on the detected operating conditions of the engine;
fuel injection amount-determining means for determining a final fuel injection amount by correcting the required fuel amount by using the calculated parameters;
engine rotational speed-detecting means for detecting an engine rotational speed of the engine; and
fuel injection timing-determining means for determining the fuel injection timing based on the detected engine rotational speed and the determined final fuel injection amount.
According to this fuel injection control system for an internal combustion engine, the required fuel amount is determined based on operating conditions of the engine, and a final fuel injection amount is determined by correcting the required fuel amount by using parameters indicative of behaviors of the fuel. Then, based on the final fuel injection amount and the engine rotational speed, the fuel injection timing is determined. Thus, the final fuel injection amount can be properly determined by using the parameters indicative of behaviors of fuel, and the fuel injection timing can be determined based on the final fuel injection amount. Therefore, differently from the prior art, it is possible to properly determine the fuel injection timing such that the fuel injection timing reflects behaviors of fuel, i.e. influence of dynamic behaviors of fuel, such as deposition of fuel, and influence of purge of evaporative fuel. This enables improvement of drivability and fuel economy.
Preferably, the fuel injection control system further includes engine temperature-detecting means for detecting a temperature of the engine, and fuel injection timing-correcting means for correcting the fuel injection timing based on the detected temperature of the engine, during the homogenous combustion mode.
According to this fuel injection control system, the fuel injection timing is corrected based on the detected temperature of the engine, during the homogenous combustion mode. Therefore, it is possible to properly set the fuel injection timing such that the fuel injection timing reflects the temperature of the engine. For instance, during the homogenous combustion mode, if the piston is cold due to a low engine temperature, in view of a high tendency for forming unatomized fuel droplets when the distance between an injector from which fuel is injected and the piston is large, by advancing the fuel injection timing, it is possible to promote atomization of fuel to thereby ensure excellent combustion of fuel.
Preferably, the fuel injection timing-determining means determines start timing of the fuel injection by calculating back the start timing from termination timing of the fuel injection calculated based on the detected engine rotational speed, by using a time period corresponding to the final fuel injection amount.
Preferably, the operating condition-detecting means includes fuel pressure-detecting means for detecting a fuel pressure of fuel to be injected into the cylinder, and the fuel injection amount-determining means includes fuel injection amount-correcting means for correcting the final fuel injection amount according to the detected fuel pressure.
Preferably, the operating condition-detecting means includes fuel pressure-detecting means for detecting a fuel pressure of fuel to be injected into the cylinder, and the parameter-calculating means includes parameter-correcting means for correcting the parameters according to the detected fuel pressure.
Preferably, the parameters indicative of behaviors of fuel are a ratio of an amount of part of injected fuel which is injected in the present combustion cycle, the part of the injected fuel being actually burned in the present combustion cycle, to an amount of the injected fuel, and a ratio of an amount of part of deposited fuel which was left deposited on inner walls of a combustion chamber defined within the cylinder at an end of the immediately preceding combustion cycle, the part of the deposited fuel being actually burned in the present combustion cycle, to an amount of the deposited.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.